Enabling ECMP support for BGP/MPLS VPNs allows multiple VPN routes to be included in the list of available equal-cost paths. You can use the maximum-paths command with the ibgp or eibgp keywords to enable ECMP support for BGP/MPLS VPNs.
The eibgp keyword specifies that the E-series router consider both external BGP (EBGP) and internal BGP (IBGP) paths when determining the number of equal-cost paths to the same destination that BGP can submit to the IP routing table. The ibgp keyword specifies that the E-series router consider multiple internal IBGP paths, but not EBGP paths, when determining the number of equal-cost paths.
Example 1
You can create an ECMP environment in which multiple IBGP paths are selected as multipaths and used for load balancing. In the example shown in Figure 93, the E-series router gives equal consideration to IBGP VPN routes learned from multiple remote PE devices when determining load balancing.
Figure 93: BGP/MPLS VPN IBGP Example
The sample BGP/MPLS network connects PE 1, PE 2, and PE 3, which are configured for VPNv4 unicast IBGP peering. CE 1 and CE 2 are configured for EBGP peering with the PE devices. CE 2 is multihomed, connected to both PE 2 and PE 3.
VRF A has two equal-cost paths through the MPLS network to get to CE 2: the IBGP path to PE 2, and the IBGP path to PE 3.
To support BGP/MPLS ECMP, PE 1 is configured with the maximum-paths ibgp command under IPv4 unicast VRF A address family. Doing this allows IBGP paths from both PE 2 and PE 3 to be selected as multipaths for use in load balancing.
Traffic from CE 1 to CE 2 that takes an IBGP route from PE 1 to either PE 2 or PE 3 is forwarded as MPLS-encapsulated packets. PE 2 and PE 3 receive the MPLS-encapsulated traffic from PE 1, remove the MPLS encapsulation, and then forward the traffic as IP packets by means of their EBGP route to CE 2.
Example 2
You can create a mixed ECMP environment in which both EBGP and IBGP paths are selected as multipaths and used for load balancing. Doing this enables the E-series router to take into account both EBGP VPN routes learned from a CE router device and IBGP VPN routes learned from a remote PE device when determining load balancing.
In Figure 94, a BGP/MPLS network connects PE 1 and PE 2, which are configured for VPNv4 unicast IBGP peering. CE 1 and CE 2 are configured for EBGP peering with the PE devices. CE 2 is multihomed, connected to both PE 1 and PE 2.
Figure 94: BGP/MPLS VPN EIBGP Example
VRF A has two paths to get to CE 2: the IBGP path through the MPLS network, and the EBGP path by means of regular IP.
To support BGP/MPLS ECMP, PE 1 is configured with the maximum-paths eibgp command in the IPv4 unicast VRF A address family. Doing this allows both the EBGP paths from CE 2 and the IBGP paths from PE 2 to be selected as multipaths in the VRF A routing information base (RIB) for use in load balancing.
Traffic taking the various routes from CE 1 to CE 2 is treated as follows:
maximum-paths
- host1(config)#router bgp 100
- host1(config-router)#address-family ipv4 vrf
vrfA
- host1(config-router-af)#maximum-paths eibgp
6
Maximum number of both EBGP and IBGP equal-cost paths is 16
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